The present invention relates to improved means and methods for providing multilayer printed circuit boards, such as are commonly employed in digital computers.
Multilayer printed circuit boards are widely used in computer systems for providing interconnections between integrated circuit (IC) chips and other components. As is well known, a multilayer printed circuit board typically comprises a board having a plurality of insulated conductive trace layers, including outer and inner conductive trace layers and power and ground planes. Components such as integrated circuits (ICs) are typically mounted on the outer board surfaces and electrically connected to trace layers formed on one or both of the outer surfaces. Interconnections between inner and outer trace layers and to power and ground planes are typically provided using plated-through holes or "vias." Some examples of prior art multilayer printed circuit boards are described and shown in U.S. Pat. Nos. 4,328,531 and 4,560,962.
One of the problems associated with the use of multilayer printed circuit boards is that, during operation, the switching of ICs can cause high speed fluctuations in the voltage level between the power and ground planes. As a result, it is typical to provide a by-pass capacitor in the general vicinity of each IC connected between the power and ground planes in order to provide voltage stabilization. Besides the undesirability of having to provide such a by-pass capacitor in the general vicinity of each IC, this by-pass capacitor introduces its own problems, particularly at high frequencies. One reason is that, whether the capacitor is of the surface-mounted or through-hole type, its structure, and the necessary connection to power and ground planes through "vias" introduces a small amount of inductance which impedes the effectiveness of its by-passing action at high frequency thereby detracting from the voltage stabilization which can be provided between the power and ground planes at high frequencies. Furthermore, such high frequency noise is a primary source of radiated energy which increases the difficulty of meeting EMI requirements. Still further, these by-pass capacitors can typically constitute 40% to 50% of the parts installed on many boards, which can have a significant impact on scheduling, purchasing, stocking, inspection, assembly, testing, and production yields.